1. Field of the Invention
The present invention relates to an intake device of an internal combustion engine.
2. Description of the Related Art
In an attempt to improve volumetric efficiency when the engine is operating under a heavy load and to create a strong swirl motion in the combustion chamber when the engine is operating under a light load, an internal combustion engine is known in which a common intake passage is provided for a first intake valve and a second intake valve, which valves are arranged adjacent to each other (see Japanese Unexamined Utility Model Publication No. 60-92733). In this engine, the ccmmon intake passage is divided into a first intake passage and a second intake passage by a dividing wall which extends to a point upstream of the common intake passage from a position between the first intake valve and the second intake valve. A separating wall projecting downward from the upper wall of the intake port is formed in the intake passage. This separating wall extends along the axis of the first intake passage from a position around the valve step of the first intake valve to the central portion of the common intake passage beyond the upstream end of the dividing wall, i.e., upstream of the intake passage. An intake control valve, which is open when the engine is operating under a heavy load, is arranged between the upstream end of the separating wall and the side wall of the common intake passage, which is located on the second intake passage side. One of the passage portions of the first intake passage, which are formed by the separating wall, has a helical shape.
In this engine, when the engine is operating under a light load, the intake control valve is closed, and at this time, a large part of the air flows into the combustion chamber via the helically-shaped passage, and thus a strong swirl motion is created in the combustion chamber.
Another engine is known in which a separating wall projecting downward from the upper wall of the intake passage and extending along the axis of the intake passage is formed in the intake passage upstream of the intake valve (see Japanese Unexamined Utility Model Publication No. 59-154826). An intake control valve is arranged in one of passage portions of the intake passage, which are formed by the separating wall.
In this engine, when the engine is operating under a light load, the intake control valve is closed. At this time, air flows within the other passage portion of the intake passage, in which the intake control valve is not arranged. This air is caused to swirl along the curved circumferential wall formed around the valve stem of the intake passage, and thus a strong swirl motion is created in the combustion chamber.
However, in the engine disclosed in the above-mentioned publication No. 60-92733, one of the passage portions of the first intake passage, which are formed by the separating wall, is a helically shaped passage having a large flow resistance, in order to create a strong swirl motion within the first intake passage. In addition, since the separating wall extends approximately over the entire length of the first intake passage, the intake passage has a large flow resistance. Consequently, in this engine, a problem occurs in that a high volumetric efficiency cannot be obtained when the engine is operating under a heavy load at a high speed.
Conversely, in the engine disclosed in the above-mentioned publication No. 59-154826, although only one intake valve is provided for each cylinder, the separating wall has a short length, and one of the passage portions of the intake passage, which are formed by the separating wall, is not a helically-shaped passage having a large flow resistance. Consequently, in this engine, a high volumetric efficiency can be obtained when the engine is operating under a heavy load at a high speed.
However, since this engine is provided with only one intake valve, it is not provided with a dividing wall, as disclosed in the above-mentioned publication No. 60-92733. Consequently, even if such a separating wall, as disclosed in the publication No. 60-92733, is applied to the construction of the intake passage disclosed in the above-mentioned publication No. 59-154826, a problem arises in that a high volumetric efficiency will not be obtained if the positional relationship between the dividing wall and the separating wall is not precise.